There have been developed recently various coatings for metallic surfaces which incorporate an organic coating resin, typically a styrenebutadiene copolymer or an acrylic polymer or copolymer, which is subsequently post-treated with a chromium containing solution to achieve corrosion resistance.
Generally, the coating is applied by immersing the metallic surface in an acidic aqueous coating composition containing the organic coating material, an oxidizing agent, and hydrogen ion. The thickness of the coating can be affected, for example, by such factors as total solids, pH, and concentration of the oxidant. Further, the thickness of the coating is a function of the coating time. The initial coating is adherent and, if desired, can be rinsed before being cured by baking. However, such a coating is incapable of providing effective corrosion resistance for the metallic substrate, as determined by standard salt spray tests.
The corrosion resistance of the prior art coatings is materially increased by rinsing the coating--prior to baking--in an aqueous chromium rinse solution, but appreciable chromium ion concentrations are required to give acceptable coatings. The necessary additional chromium rinse step is undesirable from both an economic and a pollution standpoint.
The above described coating compositions and coating and rinsing procedures are fully described in U.S. Pat. Nos. 3,585,084; 3,592,699; 3,791,431; 3,795,546; and 3,063,877.
From the present state of the art, as above described, it will be appreciated that there is a need for coating compositions and coating methods capable of producing adherent metal coatings possessing satisfactory corrosion resistance properties without requiring the additional rinse with chromium-containing rinse solutions.